Briefly, a typical prior art concentrating solar collector system comprises a concentrator having a suitable reflective surface which may be formed from a plurality of individual mirrors, a receiver for receiving or absorbing the concentrated solar radiation, and associated support structures. The solar collector system is also provided with suitable primary and secondary tracking means so that the mirrors can be made to follow the apparent movement of the sun on a daily as well as on a seasonal basis. A point-focusing solar collector system is arranged and constructed so that the sun's rays falling on the reflective surface are focused into the receiver to be utilized in any well known manner, such as for example to heat a suitable circulating fluid which can be used to power an engine or be transported elsewhere for various uses, applied directly to photovoltaic or other suitable direct energy conversion devices, applied to a solar reactor for a variety of chemical processes, and the like. The receiver can be arranged to move with the concentrator or be fixed with respect to the moving concentrator assembly.
The prior art is replete with a multitude of different designs of solar collector systems and support and tracking structures. Although ring and wheel type members have been employed in a variety of ways in the prior art, many of such prior art systems are not practicable and of those that are, all have been complex, heavy and expensive and none have been entirely satisfactory in meeting the commercial requirements of the marketplace.
German Patent No. DE-34188-879-A-1 (Mayr) is one of many examples of solar collector systems which utilize an integral arcuate ring or rings to provide for the secondary tracking motion, which rings are in turn mounted on a turntable which provides the primary tracking motion. Although the ring usually includes tension members to stabilize the ring, such ring is employed to provide for the secondary tracking motion and not the primary tracking motion. Moreover, in such system the weight is not carded by the rim but rather by hubs in a gimbal mount arrangement. In addition to carrying the weight, the hubs in the Mayr structure must also transmit the forces needed to balance the cumulative forces imparted by all the tension members. These forces, in turn, must then be externally accommodated in the turntable structure and gimbal mount which requires that they be quite heavy.
U.S. Pat. No. 4,209,231 (Sayre) employs a plurality of mirrors in a rectangular array which is diametrically mounted within a pair of circular rims which extend perpendicular to the mirror array. The rims are rotated to adjust the elevational orientation of the mirror assembly and are in turn mounted for rotation on a turntable to adjust the azimuth orientation of the mirror assembly.
Although turntables used in such prior art structures provide for distributed support to foundations, these approaches are limited to modest sizes. Turntables which do not use spokes are limited to small sizes (concentrators under 50 square meters) because of the weight of the ring required to transmit the wind forces involved in larger concentrators. Turntables which do use spokes (typically supporting concentrators under 200 square meters) are also limited by the need to prevent uplift. If uplift is prevented at the hub, very large forces are involved unless the length of the moment arm (the radius of the ring) is also made large and expensive. To prevent uplift through the connections between the turntable ring and the foundations requires elaborate mechanisms which both capture the specially shaped ring under all conditions and also allow its rotation. Because these supports interface with the circular frame in only one plane, transmitting both radial and axial loads associated with wind and gravity forces requires that they employ a plurality of supports (which for equatorial mounts in the tropics become very long and expensive) and a very substantial and expensive circular ring. In addition, these prior art approaches require complicated and expensive connections in order to accommodate the concentrated forces at any orientation and at the same time minimize the backlash without binding. An inopportune gust of wind when the concentrator is facing the horizon, for example, can pry such prior art devices off their foundation piers.
U.S. Pat. No. 4,870,949 (Butler), describes a centerless drive tracking arrangement which utilizes a drive ring to support a dish solar concentrator, which concentrator may be either fixed or pivotally mounted within the drive ring. The tracking system of the foregoing Butler patent uses a centerless drive for elevation tracking combined with centerless drive or pivoted azimuth tracking. The various rings of the various Butler configurations are independent from each other. The shape and strength of the ring used for orienting the collector system in the primary direction does not contribute to the shape and strength of the ring which supports the concentrator. This requires the expense of redundant materials. The rings used for the primary tracking motion rely simply on the strength of the rings to maintain their shape. Transmitting the forces of the wind on the concentrator to the ground results in substantial deflections which vary as a function of attitude, since these forces are carried through stress concentration regions both at the points which support the concentrator and at the interfaces with the foundations. These simple rings or hoops do not transmit either radial or axial loads without distortion unless they are made extraordinarily heavy or the concentrator area is kept small. Because the support structure moves within the primary ring, the optical effects of these structural deflections of the independent structures are cumulative.
The present invention employs one or more curved rim members in combination with other components arranged to form an open structure in a tension-compression mode which produces a very strong, stiff and lightweight tension-compression main support and tracking structure which is stable in all orientations. For example, in one embodiment the main tension-compression support structure is arranged and constructed in a bicycle wheel type tension-compression rim-spoke configuration. To the best of Applicants' knowledge none of the prior art systems and structures, including those which employ ring or wheel type members have employed such rings or wheels as in the present invention, wherein the curved rim member is arranged in combination with suitable structural means and tension members to produce a very strong and light-weight main support structure in a tension-compression mode, which can also provide for primary horizon to horizon tracking motion. The main primary tracking support structure of this invention, in addition to being very strong and light-weight, is capable of supporting large heavy loads and distributing both axial and radial loads directly to the ground through all orientations of the main support structure, and is much less expensive to manufacture and install than any other prior art structures or systems of which Applicants are aware.
Most heliostats, solar furnaces, and point-focus solar collector systems utilize a concentrator consisting of either a single monolithic reflector or one made of a static Fresnel array of mirrors. These systems follow the apparent daily and seasonal movement of the sun by moving on two axes. The reflector of both a heliostat and a solar furnace reflects sunlight to a stationary focal region. In a point focus solar collector system, the receiver moves along with the reflector. A simple prior art approach for supporting such concentrators has been the use of a single pedestal which accommodates the two drives. The cost of transmitting gravity and wind forces of the active structure through the drives and into the foundation has limited this approach to concentrators of about 100 square meters.
Point-focusing dish solar concentrators provide the highest possible optical and operating performance, high temperature capability (1500 degrees Centigrade and above), minimum land use, and a high degree of modularity (power plant sizes from kilowatts to hundreds of megawatts). Accordingly, such dish concentrator systems are very versatile and are adaptable to many near term markets for solar thermal applications, particularly electric power generation in remote and community-scale installations, as well as, providing industrial process heat, including producing high value chemicals, renewable fuels (hydrogen), and destroying toxic wastes.
It has long been recognized that the concentrator of a point-focusing solar collector system accounts for a relatively large part of the total solar collector system cost. In order to gain better economy of scale, the concentrator should be large. For a number of years the Assignee of this invention, Power Kinetics, Inc. of Troy, N.Y., has designed and built many solar collector systems which follow the daily apparent movement of the sun on one axis and adjust the angle of the mirrors of the concentrator to accommodate the other required axis of motion to adjust for the seasonal variation of the solar position. Such solar collector systems utilized large concentrators employing a plurality of dynamic Fresnel reflector elements placed with each one aimed so rays coming from the sun are reflected to the receiver. The receiver could be arranged to move with the concentrator assembly, or be fixed with respect to the moving concentrator assembly. In the foregoing arrangement mirrors or other suitable reflective elements are attached to a mirror support assembly, which mirror support assembly is arranged and constructed to rotate about an axis so as to accommodate declination (or elevation) excursions of the sun. Multiple rows of such mirror support assemblies are aligned on a frame that is arranged to track the sun in Right Ascension (where the tracking axis is the polar axis), or azimuth (where the tracking axis is vertical), or other appropriate axis. For example, although not shown in any patents of which Applicants are aware, Power Kinetics, Inc., has in the past number of years designed and built many such solar collector systems which consist of dynamic Fresnel reflectors that have the focus/receiving means fixed to and move with the concentrator and wherein tracking mounts have ranged from altitude/azimuth and modified altitude/azimuth (tilted track) to the large (300 square meter) polar axis solar collector systems, such as illustrated in FIG. 1.
Although such prior art dynamic Fresnel type concentrator solar collector systems have operated entirely satisfactorily with respect to the ability to concentrate and collect solar energy, they have remained too expensive to manufacture and erect to be entirely acceptable for many promising near term commercial applications. For example, such systems require large piers to support the large concentrator, involve many foundations for both support piers and guy cables, usually require heavy equipment, such as cranes, for installation at the site and, where the receiver is large and heavy, such as for example in the case where the receiver is a large boiler or an engine, it is difficult even with the use of heavy and expensive structural members to achieve the desired rigid support for such receiver as the concentrator is moved to track the daily apparent movement of the sun. To prevent distortion, the concentrator frame must be made very strong and rugged which requires the use of heavy and expensive structural members. Very high performance components are required for the Right Ascension drive to handle the large forces without excessive lost motion. To prevent damage from typhoons or hurricanes when not in operation a separate stow locking system is required. Further, since the structure is suspended between two piers, all forces must be transmitted through the shafts and bearing connections at both ends of the central structural beam, and the concentrator assembly must be rigid enough at any attitude to accommodate dynamic wind gusts from any direction without inordinate distortion. Maintaining accurate tracking in windy regions is difficult.
It is also well known, that equatorial mounted, polar tracking solar collector systems are desirable since they have the advantage of a constant tracking rate at the earth's rotational speed (15 degrees per hour) and a slow seasonal movement about the declination axis (a maximum of 0.0163 degrees per hour at the equinoxes and zero at the solstices). Prior to the present invention, however, the advantage of this simple tracking has been offset by several disadvantages. One such disadvantage is that the mass of the concentrator must be supported on a tilted axis resulting in high thrust loading and moments which have required heavy and expensive supporting structures. Another is that the weight of the receiver distorts the concentrator frame to an increasing degree as the collector rotates east or west from the noon position in which the receiver is directly above the axis of rotation. Still further, with such prior art solar collector systems, it is difficult to design an accurate, low power, and inexpensive tracking mechanism, and one where the receiver and associated hardware can be easily lowered for maintenance. In addition, in many arrangements a plurality of rotatable unions or flexible connections are required between the receiver and the ground which further contributes to the cost.
Accordingly, there remains a continuing need to provide improved solar collector systems which are less expensive to manufacture and install, as well as the need for less expensive and lightweight tracking structures for such systems which can accommodate large concentrators while providing for rigidity, accurate tracking using a minimum of power, and be capable of withstanding or avoiding damage from the effects of severe weather and environmental conditions, including high winds, hail, ice, snow, and the like.
To be commercially acceptable for a wide range of commercial applications a solar radiation collector system should:
be economical to manufacture and install; PA1 employ a support structure which is simple, strong, lightweight, and capable of supporting large concentrators, and also heavy receivers when required; PA1 be able to withstand extreme weather conditions, including severe winds, hail, ice, snow, and the like; PA1 provide accurate tracking with low power requirements; PA1 be easily modified to place the center of gravity on the axis of rotation for a variety of applications;
The present invention provides a new and improved tracking approach which allows a tracking solar collector system to achieve all of the foregoing desiderata, and overcomes one or more of the other problems and disadvantages of the known prior art systems.